Catheters represent the most common medical devices for introduction into a patient's body. A catheter, in general, is a tubular medical device for insertion into canals, vessels, passageways, or body cavities usually to permit injection or withdrawal of fluids or to keep a passage open or to carry sensors. Catheters are often applied for determining blood flow and/or volumetric hemodynamic parameters with thermodilution techniques, or for invasive blood pressure and/or oxygen saturation measurements with embedded fiber optics.
Thermodilution catheters, as typical examples for catheters, are well known in the literature and medical practice for determining blood flow, cardiac blood volumes, or pulmonary blood volumes. Such catheters are typically small diameter catheters for insertion in blood vessels, carrying distal temperature sensing means with more or less additional lumens, to inject the dilution liquid and/or to transmit a blood pressure in a vessel at a distal opening to a proximal opening for pressure sensing means. A common characteristic of the thermodilution measurements is the injection of a cold bolus and to sense the temperature displacement downstream with the temperature sensing means of the catheter. From the magnitude, duration, appearance time, mean transit time, and down slope time of the temperature displacement curve, flow and volume parameters of organs or parts thereof between site of bolus injection and site of temperature sensing are derived by appropriate measurement algorithms of a general purpose processing device. A representative thermodilution catheter is illustrated in U.S. Pat. No. 3,995,623.
FIG. 1 illustrates a conventional thermodilution catheter 2 for insertion by central venous access through the right heart into the pulmonary artery. The catheter 2 comprises a distal tip orifice 1, communicates through an internal lumen of the catheter 2, and terminates in a luer fitting 3. A balloon 4 communicates through an internal lumen of the catheter 2 and terminates in a syringe 5 for the purpose of inflating the balloon. A thermistor 6 is connected by small wires embedded in a wall of catheter 2 to dedicated pins in a catheter connector 7. An injectate orifice 8 communicates through an internal lumen of the catheter 2 and terminates in a luer fitting 9.
FIG. 2 illustrates a conventional thermal dilution catheter 2 for insertion by an arterial access into an aortic or near an aortic vessel. The catheter 2 comprises the same characteristics as in FIG. 1, however, without the balloon 4, the syringe 5 including internal lumen, the injectate orifice 8, and luer fitting 9 including internal lumen.
FIGS. 3A and 3B illustrate in detail an example of the electrical catheter connector 7 as used e.g. by the Hewlett-Packard HP M1642A Catheter Interface Cable. The lug of the catheter connector 7 in FIG. 3A comprises electrical contact pins 10 enclosed by a threaded collar 11. A corresponding device connector socket 7A in FIG. 3B is adapted to receive the catheter connector plug of FIG. 3A. In FIG. 3B, a device connector 14 includes electrical pin receptors 10A and a threaded slip collar 15 to secure the connection of the connectors 7 and 14.
Thermodilution catheters today are used most widely to determine blood flow and pressures in the pulmonary artery. Such catheters are inserted via a central venous access like the right internal jugular vein or left subclavian vein through the right heart and placed with the distal end in the pulmonary artery. The rate of blood flow is computed from the displacement of blood temperature according to the Stewart-Hamilton dilution equation for a thermal indicator as described in U.S. Pat. No. 3,987,788 or in the publication "The thermodilution method for the clinical assessment of cardiac output", J. R. C. Jansen, Intensive Care Med (1995) 21:691-697. In another application, the transpulmonary thermodilution technique, the thermodilution catheter is located in the arterial side of the vascular system and placed via a femoral, radial or axillary artery access. In addition to the derived blood flow calculation the circulatory filling status can be determined from the appearance time, mean transit time and down slope time of the blood temperature displacement as described in the U.S. Pat. No. 5,526,817.
It has been shown that erroneous applications of catheters, e.g. due to a wrong placement in the intra-vascular system or selection of the measurement parameters, can lead to wrong measurement results and might cause serious harm to the patients, e.g. due to a wrong therapeutic decision based on the wrong results.